Vibratory gyroscopes provide a measure of the rate of rotation by sensing the effects of a Coriolis force on an oscillating body. Such sensors are very interesting for a number of applications. Though lacking the precision of the rotary gyros, their price makes them attractive for many applications. One example is the automotive brake control system, where the rate of rotation of the car needs to be sensed and controlled to avoid spin. Prices of many vibratory solid state gyros, using either quartz or piezo-electric materials are at present in the $500 to $1500 range.
This price is excessive for many potential mass applications such as automotive brake systems, robotic control, patient monitoring, virtual reality simulation, video games etc. These uses need another one or two orders of magnitude in price reduction to make the product viable and require semiconductor fabrication techniques to achieve this price performance ratio.
Micromachined rate gyro sensors have been made in the past. U.S. Pat. No. 4,598,585, by B. Boxenhorn, assigned to Draper Laboratory, describes a micromachined planar inertial sensor, consisting of a pair of gimbals, positioned at right angles to each other. The inner gimbal plate carries on it a substantial mass, which acts as the gyroscopic detector. The outer gimbal, noted as the y-axis in the patent, is driven by electrostatic forces (or electromagnetic forces), and is oscillating in a torsion mode, at a frequency equal to the torsional resonance frequency of the inner gimbal. Rotation of the sensor around the z axis causes the first oscillation to excite the inner resonance frequency, which is detected by a set of capacitive sensors on the inner gimbal.
The method is elegant in principle. In prior art devices, it has been suggested that the gimbals may be made out of many materials, such as silicon dioxide, nitride, oxy-nitrides, or even stamped steel or aluminum sheets. During their deposition it is very difficult to produce materials with the right stress. As a result, the frequency of the inner gimbal is not well determined and needs to be trimmed, in order to match the driving frequency. These materials are also subject to work hardening, hence the frequency of the inner resonance will change over time, causing a mismatch with the driving frequency, and an apparent loss of sensitivity.
U.S. Pat. No. 4,699,006 by B. Boxenhorn discloses a vibratory digital integrating accelerometer, based on the same technology. In this case a z axis acceleration causes a change in the resonant frequency around the y axis. The changes in frequency are representative of the z axis acceleration.
U.S. Pat. No. 5,016,072 by Paul Greiff describes further improvements on the technique. The dielectric layers of U.S. Pat. No. 4,598,585 have been replaced with a sheet of boron doped p+ silicon, and the asymmetric mass has been replaced by a symmetric one. Buckling of the oxide inner flexures causes undesirable large variations in the inner resonant frequency; special flexure footings need to be provided. Flexure grooves are needed to give controllable stiffness in the flexure. The stress in the boron doped material requires stress relief and trimming of the hinges. Electrostatic balanced force techniques are used to restrain the motion of the inner gimbal, to avoid cross-coupling and changes of its resonant frequency. The outer axis needs to be driven at the resonance frequency of the inner axis, which is done by dead reckoning, and requires frequency trimming.
U.S. Pat. No. 5,203,208 by J. Bernstein also assigned to Draper Lab, describes a symmetric micromechanical gyroscope, also using boron doped silicon. Here the resonance frequencies of both axes are designed to be the same, and trimmed to be identical. Trimming slots are also required to relieve stress in the boron doped silicon. As a result the drive voltage can be vastly reduced, which substantially helps in the elimination of parasitic pick-up signals.
Boxenhorn and Greiff in Sensors and Actuators, A21-A23 (1990) 273-277 have described an implementation of a silicon accelerometer, of the type described in U.S. Pat. No. 4,598,585, mentioned above. Here the flexures are made out of boron diffused silicon. They ascribe the difficulties encountered with this approach due to the unknown pre-stress, which sets the torsional stiffness and sensitivity of the device.
In all the above processes, the stresses in the hinge material are uncontrolled, and the frequencies unpredictable. An object of the invention was to devise a low-cost micromachined gyroscope having improved frequency stability and which is easy to manufacture.